1. Field of the Invention
The present invention relates to an oxygen concentration detecting device, and in particular, to an insulating layer formed on the measuring electrode of the oxygen concentration detecting device.
2. Description of the Prior Art
An oxygen concentration detecting device comprises a solid electrolyte, a pair of electrodes sandwiching the solid electrolyte and an insulating layer covering one of said electrodes contacting exhaust gas.
In order to obtain an oxygen concentration detecting device, it has conventionally been carried out to simultaneously sinter after stacking a solid electrolyte, electrodes and an insulating layer on each other because such a production method is simple and a high adhesive force can be obtained easily.
However, in the conventional method an insulating layer made of a partially stabilized zirconia containing monoclinic and tetragonal phases, which is the same material as the solid electrolyte, is provided in order to obtain a high adhesive force, however in this method, since the insulating layer is a rough and porous layer having a lower density than the solid electrolyte, micro cracks are generated in the insulating layer when a phase transition between monoclinic phase and tetragonal phase occurs due to the volume change at the phase transition, which is characteristic to zirconia, so that there has been a problem in that the mechanical strength of the insulating layer lowers remarkably, to the extent that it exfoliates at several tenth to several hundredth cycle in a cold-hot durability test wherein the maximum temperature is 1000.degree. C.
Because of this, as disclosed in Japanese Patent Application Laid Open No. 259952/1985, it was proposed to use not the same material as the solid electrolyte but a completely stabilized zirconia composed only of cubic phase for the insulating layer.
However, when the insulating layer material of Japanese Patent Application Laid Open No. 259952/1985 is applied, since the thermal expansion coefficient of the completely stabilized zirconia as an insulating layer material is not less than 10.times.10.sup.-6 /.degree. C. while that of a monoclinic phase-containing partially stabilized zirconia as a solid electrolyte is generally 7-9.times.10.sup.-6 /.degree. C., it has been found that exfoliation occurs due to the difference in thermal expansion coefficient, which gives rise to a problem in durability.
The present invention has been achieved in view of the above problem, and an object of the invention is to provide an oxygen concentration detecting device which is excellent in durability and mechanical strength. Another object of the invention is to provide a method for producing an oxygen concentration detecting device which is excellent in durability and can be produced easily.